U.S. patent application number 15/989510 was filed with the patent office on 2018-12-06 for piston slide valve.
This patent application is currently assigned to Rausch & Pausch GmbH. The applicant listed for this patent is Rausch & Pausch GmbH. Invention is credited to Sebastian BAHR, Werner DOHLA.
Application Number | 20180347722 15/989510 |
Document ID | / |
Family ID | 62455411 |
Filed Date | 2018-12-06 |
United States Patent
Application |
20180347722 |
Kind Code |
A1 |
BAHR; Sebastian ; et
al. |
December 6, 2018 |
PISTON SLIDE VALVE
Abstract
An electromagnetically actuated piston slide valve includes a
piston slide arrangement with a piston which is axially
displaceable for regulating a free cross section of a fluid passage
of the valve. The piston slide arrangement contains a first
magnetic armature connected to the piston and a second magnetic
armature which is axially displaceable with respect to the piston.
The piston is axially displaceable against the force of a first
biasing spring by generating an electromagnetic field through
energizing a coil. A second biasing spring rests against the first
magnetic armature and the second magnetic armature, so, in the
unenergized state of the coil, the piston takes a predetermined
position by axial displacement due to the force of the first
biasing device against the force of the first biasing device. A
permanent magnet generates an attractive force between the first
and the second magnetic armature which counteracts the second
biasing spring.
Inventors: |
BAHR; Sebastian; (Selb,
DE) ; DOHLA; Werner; (Selb, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rausch & Pausch GmbH |
Selb |
|
DE |
|
|
Assignee: |
Rausch & Pausch GmbH
Selb
DE
|
Family ID: |
62455411 |
Appl. No.: |
15/989510 |
Filed: |
May 25, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F16K 31/082 20130101;
F16K 31/0668 20130101; F16K 31/0675 20130101; F16F 2230/24
20130101; F16K 31/0693 20130101; F16F 9/34 20130101 |
International
Class: |
F16K 31/06 20060101
F16K031/06; H01F 7/08 20060101 H01F007/08; H01F 7/02 20060101
H01F007/02; H01F 7/16 20060101 H01F007/16 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2017 |
DE |
10 2017 111 726.1 |
Claims
1. An electromagnetically actuated piston slide valve, comprising:
a valve housing with a first fluid connector and a second fluid
connector and at least one fluid passage connecting the two fluid
connectors, and a piston slide arrangement with a piston which is
axially displaceable in the valve housing for regulating a free
cross section of the fluid passage, a first magnetic armature
connected to the piston and a second magnetic armature which is
axially displaceable with respect to the piston, as well as a first
biasing device and a second biasing device, wherein the piston is
axially displaceable against the force of the first biasing device
by generating an electromagnetic field through energizing a coil,
and wherein the second biasing device rests against the first
magnetic armature and the second magnetic armature, so that, in the
unenergized state of the coil, the piston takes a predetermined
position by axial displacement due to the force of the second
biasing device against the force of the first biasing device, and
wherein the piston slide valve further comprises a permanent magnet
which acts in such a manner on at least one of the first magnetic
armature and the second magnetic armature that a magnetic force
caused by the permanent magnet counteracts the force of the second
biasing device.
2. The piston slide valve according to claim 1, wherein the
permanent magnet is arranged such that the magnetic force caused by
the permanent magnet moves the first magnetic armature and the
second magnetic armature towards each other.
3. The piston slide valve according to claim 1, wherein the
permanent magnet is arranged such that it exerts an attractive
force between the first magnetic armature and the second magnetic
armature.
4. The piston slide valve according to claim 1, wherein the
permanent magnet is arranged between the first magnetic armature
and the second magnetic armature.
5. The piston slide valve according to claim 1, wherein the
permanent magnet is arranged, in particular embedded or inserted,
in at least one of the first magnetic armature and the second
magnetic armature, preferably in the first magnetic armature.
6. The piston slide valve according to claim 5, wherein the
permanent magnet is arranged in at least one of the first magnetic
armature and the second magnetic armature near a surface which
faces the correspondingly other one of the first and the second
magnetic armature.
7. The piston slide valve according to claim 1, wherein the
permanent magnet is magnetized in the axial direction.
8. The piston slide valve according to claim 1, wherein the
magnetic force caused by the permanent magnet is weaker than the
force of the second biasing device.
9. A piston slide valve according to claim 1, wherein the permanent
magnet is a ring magnet.
Description
FIELD
[0001] The present invention relates to a piston slide valve, for
example for a shock absorber of a vehicle.
BACKGROUND
[0002] An electromagnetically actuated piston slide valve can be
used as a throttle valve in a hydraulic shock absorber of a vehicle
in order to adjust a shock-absorber characteristic to be "hard" or
"soft". By means of the adjustable throttle valve the flow
resistance of the valve and thereby the shock-absorbing effect of
the entire system can be changed in dependence on the electrical
energizing of the field coil of the valve. The valve connects two
shock-absorber chambers here, wherein pressure surges on the shock
absorber cause a fluid displacement from one shock-absorber chamber
into the other shock-absorber chamber.
[0003] In dependence on the application, it may be required that
the valve is closed ("normal closed", NC) or open ("normal open",
NO) in the currentless state. When the valve takes a predetermined
position in the currentless state, this is also referred to as
fail-safe state, since the valve takes this state when the entire
system is turned off or fails, for example when the power supply
breaks down. This fail-safe function is used in shock-absorbers for
motor vehicles, for example. It can be advantageous when the
fail-safe state defines a partly opened state of the valve, so that
in the case of a system failure the shock absorber does not switch
to a very soft or hard setting, in order to ensure a moderate and
secure driving condition thereby.
[0004] From DE 10 2008 035 899 A1 and DE 10 2013 106 214 A1
electromagnetically actuated NO valves are known. These valves have
a fail-safe position in which the valve is partly opened, i.e. a
position between the maximally opened and closed the position in
the unenergized state. When the coil of the valve is electrically
energized, the piston (also referred to as slide) of the valve
initially moves to the maximally opened position and can be held
there with a basic energy supply, i.e. a minimum energy supply that
is required for keeping the valve maximally open. When the current
is further increased, the slide moves continuously in the direction
of the closed position.
[0005] These valves have two magnetic armatures and two
corresponding biasing springs. In order to hold the valve in the
maximally opened position, a minimal magnetic force is necessary
and in a "basic energy supply" in order to overcome the force of
the fail-safe spring, i.e. that spring which urges the piston in
the direction of the fail-safe position. When the energy supply is
lowered below the basic energy supply, the valve switches to the
fail-safe state. A lowering of the basic energy supply is thus
impossible. Also external influences, such as vibrations due to
unevenness of the road surface, can likewise have the result that
with basic energy supply the valve unintentionally switches from
the maximally opened position into the fail-safe position.
Moreover, the fail-safe spring cannot be configured with any
desired stiffness, and thus the fail-safe stroke cannot be
configured with any desired dimension, since the valve would
otherwise switch to the fail-safe state too easily. Put
differently, a stiff fail-safe spring requires a high basic energy
supply for overcoming the biasing force of the fail-safe spring and
for holding the maximally opened position of the valve.
SUMMARY
[0006] It is therefore the object of the invention to make
available an electromagnetically actuated piston slide valve which
has a fail-safe function at low electric basic electrical energy
supply, a large fail-safe stroke of the valve and stability against
external disturbing influences.
[0007] A piston slide valve according to the invention comprises a
piston slide arrangement with a piston which is axially
displaceable in a valve housing for regulating a free cross section
area of a fluid passage between a first fluid connector and a
second fluid connector of the valve. Further, the piston slide
arrangement comprises a first magnetic armature connected to the
piston and a second magnetic armature which is axially displaceable
with reference to the piston, as well as a first biasing device and
a second biasing device. In particular, the first and the second
biasing device can be a biasing spring in each case. The first
magnetic armature is connected to the piston, i.e. it moves axially
together with the piston. In particular, the connection can be
referred to as "permanent" or "stationary", wherein the first
magnetic armature and the piston do not necessarily have to be
interconnected inseparably.
[0008] By generating an electromagnetic field by energizing a coil,
the piston is axially displaceable against the force of the first
biasing device. At the same time, the second biasing device rests
against the first magnetic armature and the second magnetic
armature, so that, in the unenergized state of the coil, the piston
takes a predetermined position through axial displacement due to
the force of the second biasing device against the force of the
first biasing device. The predetermined position defines the
fail-safe position of the valve explained at the outset and is
taken in particular during the transition from the energized state
to the unenergized state of the valve, i.e. in particular when the
valve is turned off or upon a system failure. The piston slide
valve according to the invention can be used advantageously in a
shock absorber for a motor vehicle.
[0009] According to the invention, the piston slide valve further
comprises a permanent magnet, which acts on at least one of the
first magnetic armature and the second magnetic armature such that
a magnetic force caused by the permanent magnet counteracts the
force of the second biasing device. The magnetic force acts in
particular in an energized state of the valve, in which the current
intensity is larger or equal to a basic energy supply. When the
term "magnet" is used in the following description, the permanent
magnet is meant. In the event that the electromagnet, i.e. in
particular the coil, of the valve is meant, this will be specified
explicitly. The basic energy supply defines the minimum current
intensity which is necessary for overcoming the force of the second
biasing device and for example for holding a maximally opened
position of the valve.
[0010] By providing a permanent magnet counteracting the force of
the second biasing device the disadvantages described at the outset
can be overcome. In particular, the permanent magnet permits a
lower basic electrical energy supply of the valve, since it
supports the position, e.g. the maximally opened position, reached
through the basic energy supply of the valve, meaning that it
contributes to holding this position without the valve switching
back to the fail-safe position unintentionally. A substantially
reduced energy consumption can be achieved thereby, since several
valves, for example up to eight valves, can be installed in a
vehicle. In addition, a harder fail-safe coordination becomes
possible, i.e. a larger fail-safe stroke of the slide or piston.
The piston slide valve according to the invention with the position
of the valve achieved through the basic energy supply has an
increased robustness with respect to disturbing influences, e.g.
mechanical excitation of the valve from the outside or flow forces.
Overall, an expansion of the regulation range of the vale with a
better resolution can be achieved thereby.
[0011] Preferably, the permanent magnet is arranged such that the
magnetic force caused by the permanent magnet moves the first
magnetic armature and the second magnetic armature towards each
other. In particular, the permanent magnet can be arranged such
that it exerts an attractive force between the first magnetic
armature and the second magnetic armature. This can be the case in
particular when the second biasing device is arranged such that it
pushes the first magnetic armature and the second magnetic armature
apart. The magnetic force of the permanent magnet supports the
holding force between the first and the second magnetic
armature.
[0012] As already indicated briefly, the described effect of the
magnetic force generally acts in particular on a state of the valve
in which at least the basic energy supply is applied. In other
words, the magnetic force is described for a state of the valve in
which the first magnetic armature and the second magnetic armature
are arranged sufficiently close to each other, in particular are
directly adjacent to each other, and optionally touch each other.
This position of the valve is achieved in particular by applying
the basic energy supply, which overcomes the force of the second
biasing device, i.e. in particular the fail-safe spring, which
rests against the first and the second magnetic armature. In
contrast, the first magnetic armature and the second magnetic
armature are possibly too far away from each other in the
unenergized state, so that the permanent magnet cannot develop a
relevant force between the first magnetic armature and the second
magnetic armature, i.e. in particular no or only a very small
force, which counteracts the force of the second biasing
device.
[0013] The permanent magnet can be arranged in particular between
the first magnetic armature and the second magnetic armature.
Advantageously, the permanent magnet is arranged in at least one of
the first magnetic armature and the second magnetic armature. The
magnet can be arranged in the first magnetic armature, for example.
It is understood that the magnet can also be arranged in the second
magnetic armature alternatively. It is also conceivable that at
least one permanent magnet is provided in both the first and the
second magnetic armature. These can be arranged for example in the
radial direction in mutually offset manner, or in axially mutually
aligned manner, taking account of the direction of the
magnetization, so that a force is reached by the permanent magnets
which counteracts the force of the second biasing device.
[0014] For example, the permanent magnet can be inserted or
embedded in the corresponding magnetic armature, e.g. mounted as a
separate component or molded in as well in the injection molding
process. By arranging the magnet in the magnetic armature the
construction size of the valve remains unchanged and no additional
construction space is necessary. A further advantage of positioning
the magnet in the magnetic armature is that in the case of an
arrangement in the magnetic armature--thus on a small diameter--a
relatively small volume of the permanent magnet is already
sufficient to raise the holding force significantly.
[0015] By arranging the magnet in one of the magnetic armatures (or
possibly in both magnetic armatures) a magnetic circuit is closed
around the magnet that encompasses both armatures. It is thus
ensured that the magnet contributes only to an increase of the
attractive force between the magnetic armatures. Positioning in a
different place in the magnetic circuit (e.g. in a transfer disk)
would result in a magnet-driven flow, which, in addition to the gap
between the two magnetic armatures, passes also the operation air
gap between the first magnetic armature and a stationary pole part.
This would increase the magnetic force during regular operation in
addition to the holding force between the magnetic armatures. The
force of the first biasing spring would have to be increased
accordingly. To avoid reducing the fail-safe stroke, the fail-safe
spring force would have to increase simultaneously in this case.
The increase in holding force would be compensated thereby.
[0016] Advantageously, the permanent magnet is arranged in at least
one of the first magnetic armature and the second magnetic armature
near a surface which faces the correspondingly other one of the
first and the second magnetic armature. As mentioned, the magnet
can be inserted in the corresponding magnetic armature, for example
inserted in a recess in the surface of the magnetic armature. It
would be theoretically conceivable that the permanent magnet is
embedded in the corresponding magnetic armature near the surface,
so that the magnet is enclosed completely by the material of the
magnetic armature. However, the complete embedding of the permanent
magnet impairs the formation of the magnetic flux on the contact
area of the two magnetic armatures. Therefore, the permanent magnet
is preferably at least partly exposed.
[0017] Advantageously the permanent magnet is magnetized
substantially in the axial direction, so that it can optimally
counteract an axial force of the second biasing device. In other
words, the permanent magnet is magnetized preferably in a direction
parallel to the force of the second biasing device. Preferably, the
magnetic force caused by the permanent magnet is weaker than the
force of the second biasing device, so that the force of the second
biasing device in the unenergized state of the valve exceeds the
magnetic force and the piston can take the predetermined position,
i.e. the valve can take the fail-safe position. However, the
magnetic force of the permanent magnet is sufficiently high to
support the position of the valve during basic energy supply, as
explained.
[0018] The permanent magnet can be configured as a ring magnet or
at least comprise a ring magnet. It is understood that also other
shapes of the magnet are conceivable, for example ring magnet
segments, or other non-annular elements. The permanent magnet can
be configured in single-piece manner or can comprise several parts.
For example, several single magnets can be arranged around the
longitudinal axis of the magnetic armature, in particular arranged
regularly. The single magnets can have any desired suitable shape
in this case.
[0019] In order to further optimize the holding force between the
first magnetic armature and the second magnetic armature during
basic energy supply of the valve, a step or recess can be provided
on a front side of the first or second magnetic armature, which,
during basic energy supply of the valve, is adjacent to a
corresponding front side of the correspondingly other one of the
first and second magnetic armature. Thereby the contact area
between the two magnetic armatures is reduced and the magnetic flux
is concentrated to the relatively small contact area, resulting in
an increased magnetic flow density and thereby an increased holding
force between the two magnetic armatures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The invention will hereinafter be described with reference
to the attached drawings. The drawings are merely schematic
representations and the invention is not limited to the specific
represented embodiment examples. The valve according to the
invention is represented in FIG. 3 in particular. FIGS. 1 and 2
show known valves, which do not contain all features of the
invention, but are described in order to explain the valve
according to the invention.
[0021] FIG. 1 shows a sectional representation of a NO piston slide
valve in the unenergized state.
[0022] FIG. 2A shows a sectional representation of a NO piston
slide valve with a fail-safe function in the unenergized state.
[0023] FIG. 2B shows a sectional representation of the piston slide
valve of FIG. 2A in the opened state during basic energy
supply.
[0024] FIG. 3 shows a sectional representation of a piston slide
valve according to the invention in the opened state during basic
energy supply.
[0025] FIG. 4 shows a sectional representation of a detail of the
piston slide valve of FIG. 3.
[0026] FIG. 5 shows the detail of the piston slide valve according
to FIG. 4, including magnetic field lines.
DETAILED DESCRIPTION
[0027] In FIG. 1 a known piston slide valve 1 is represented in a
sectional view, in order to briefly explain the operation mode in
principle. In the unenergized state represented in FIG. 1 the valve
1 is opened, i.e. this is a piston slide valve 1 of the NO
construction type ("normal open"). The valve 1 has a valve housing
1 with a first fluid connector 3, which can be a fluid inlet
depending on the application, and a second fluid connector 4, which
can be a fluid outlet depending on the application. In this
embodiment example the fluid inlet 3 is arranged axially and the
fluid outlet 4 comprises several radial openings. A piston 5, which
can also be referred to as slide or piston slide and can be
configured to be hollow in particular, is arranged in axially
displaceable manner in the valve housing 2, in order to open and
close a fluid passage 6 between the first fluid connector 3 and the
second fluid connector 4, more exactly to regulate a free cross
section of the fluid passage 6. Such a slide is known for example
from EP 1 538 366 A1. It is understood that the present invention
is not limited to such a slide configuration, but that also
different slide or piston constructions can be employed for
regulating a free cross section of a fluid passage.
[0028] The piston 5 is connected to a magnetic armature 7, so that
the piston 5 and the magnetic armature 7 move together. The
magnetic armature 7--and thus the piston 5--is axially displaceable
by means of the magnetic field generated by a coil 8. When the coil
8 is energized, a magnetic force acts in a closed magnetic circuit,
and the piston 5 is moved thereby against the force of a biasing
spring 9 in the direction of a stationary pole part 17. The biasing
spring 9, which can also be referred to as regulating spring, rests
against the valve housing 2 and the piston 5 and in the embodiment
example shown here forces the piston 5 into a position in which the
fluid passage 6 is maximally opened. In other words, in the
unenergized state of the coil 8 of the valve 1 the fluid passage 6
is maximally opened, i.e. the valve 1 is normal open (NO).
Alternatively (not represented), the piston 5 could also be forced
by means of the biasing spring 9 into a position in which the fluid
passage 6 is closed (normal closed, NC).
[0029] FIGS. 2A and 2B show a piston slide valve 1', which is
constructed in principle similarly to the piston slide valve 1
shown in FIG. 1, hence the same reference numerals are employed for
corresponding parts. However, the piston slide valve 1' shown in
FIGS. 2A and 2B has a so-called fail-safe function. In this
embodiment example, the valve 1 takes a position in the unenergized
state in which the fluid passage 6 is neither maximally opened nor
completely closed, but partially opened. When used in a shock
absorber of a motor vehicle, a shock-absorbing characteristic is
reached thereby which is neither completely hard nor completely
soft, so that the vehicle has a moderate shock-absorbing
characteristic for example in the event of a failure of the
system.
[0030] The fail-safe function is achieved by a bisection of the
magnetic armature and by providing a second biasing spring in
addition to the regulation spring 9. The second biasing spring 10
is arranged between the first magnetic armature 11, which can be
referred to as regulation armature, and the second magnetic
armature 12, which can be referred to as fail-safe armature, and
pushes the two magnetic armatures 11, 12 apart from each other. The
first magnetic armature 11 is permanently connected to a piston rod
13 of the piston 5, whereas the second magnetic armature 12 is
axially displaceable on the piston rod 13. In the unenergized state
of the valve 1' represented in FIG. 2A, the force of the second
biasing spring 10 counteracts the force of the first biasing spring
9 until an equilibrium of forces is reached and the valve 1' thus
takes the partially opened position shown in FIG. 2A.
[0031] When the coil 8 of the valve 1' is energized now, as of
reaching a basic energy supply the force of the second biasing
spring 10 will be overcome by an electromagnetic attractive force
between the two magnetic armatures 11, 12, so that the two magnetic
armatures 11, 12 adjoin each other at the front sides. During basic
energy supply, the fluid passage 6 is maximally opened, as
represented in FIG. 2B. When the current intensity is increased
further, the first magnetic armature 11 and the second magnetic
armature 12 move as one unit against the force of the first biasing
spring 9 and the valve 1' is closed (not represented).
[0032] When the system is turned off or fails, i.e. when the energy
supply is turned off or fails, the second biasing spring 10 pushes
the first and second magnetic armature 11, 12 apart again against
the force of the first biasing spring 9, so that the valve takes
the position shown in FIG. 2A, i.e. the fail-safe position.
However, the valve 1' is not intended to take the fail-safe
position unintentionally, for example due to vibrations when the
road surface is uneven. Therefore, a sufficient basic energy supply
is required to hold the position shown in FIG. 2B. This means that
sufficient electromagnetic force has to be generated in order to
overcome the force of the second biasing spring 10 and to achieve a
sufficient force surplus, i.e. holding force, between the first and
the second magnetic armature 11, 12.
[0033] In FIG. 3 a piston slide valve 1'' according to the
invention is represented. The position of the valve 1'' represented
in FIG. 3 corresponds to the position of the valve 1' represented
in FIG. 2B. Reference is made to the description of FIG. 2B in this
regard, and the same reference numerals are employed for
corresponding parts. In contrast to the piston slide valve 1'
represented in FIGS. 2A and 2B, the valve 1'' according to the
invention represented in FIG. 3 has a permanent magnet 14, which
contributes to holding the maximally opened position of the valve
1'' represented in FIG. 3. For this purpose the permanent magnet 14
is arranged between the first magnetic armature 11 and the second
magnetic armature 12 to generate an attractive force between the
first and the second magnetic armature 11, 12 which counteracts the
force of the second biasing spring 10.
[0034] In particular in the state of the valve 1'' represented in
FIG. 3, i.e. during basic energy supply, in which the first and the
second magnetic armature 11, 12 adjoin each other, the magnetic
attractive force generated by the permanent magnet 14 is reached
between the first and the second magnetic armature 11, 12. In
contrast, in the unenergized state of the valve 1'' (see FIG. 2A),
in which the two magnetic armatures 11, 12 are mutually spaced
apart and pushed apart by the second biasing spring 10, no or only
a very small attractive force between the first and the second
magnetic armature 11, 12 is achieved by the permanent magnet 14,
since the attractive force strongly decreases in line with a
growing spacing of the magnetic armatures 11, 12. The
characteristic of the second biasing spring 10 in contrast is
substantially linear to the spacing of the two magnetic armatures
11, 12.
[0035] As represented in FIG. 3, the permanent magnet 14 is
arranged in the first magnetic armature 11. For example, the
permanent magnet 14 can be configured as a ring magnet, which is
inserted in a correspondingly annular recess 15 on a front side 16
of the first magnetic armature 11 (see also FIG. 4). However, as
explained above, the permanent magnet 14 can have any other desired
shapes and can be inserted or embedded in the magnetic armature 11.
The arrangement of the permanent magnet within the magnetic
armature 11 has several advantages. On the one hand, a magnetic
circuit is closed encompassing both magnetic armatures 11, 12, so
that the magnet 14 only increases the attractive force between the
two magnetic armatures 11, 12, but does not interfere with the
electromagnetic circuit and for example also increase the
attractive force with respect to the pole part 17. This would
require disadvantageously a stronger regulation spring 9 and thus a
higher basic energy supply for reaching the maximally opened
position of the valve.
[0036] The attractive force between the first and the second
magnetic armature 11, 12 generated by the permanent magnet 14
counteracts the force of the second biasing spring 10 and increases
the holding force between the first magnetic armature 11 and the
second magnetic armature 12 generated by the basic energy supply.
The basic energy supply necessary for holding the maximally opened
position of the valve 1'' can therefore be reduced in comparison to
a valve without the permanent magnet 14 (for example the valve 1'
shown in FIGS. 2A and 2B).
[0037] In the detail of the valve 1'' of FIG. 3 shown in FIG. 4 a
further measure is recognizable which can improve the holding force
between the first magnetic armature 11 and the second magnetic
armature 12. The first magnetic armature 11 has a shallow step 18
on its front side 16 facing in the direction of the second magnetic
armature 12. Put differently, the front side 16 of the first
magnetic armature has a slightly backwardly offset region 19 which
can be a radially inner region in particular. For this reason,
there is a direct contact between the first magnetic armature 11
and the second magnetic armature 12 only at an outer edge. In this
manner, the magnetic flux is concentrated to a small contact area
20, so that the magnetic flux density increases there, which, in
turn, generates an increased holding force between the two magnetic
armatures 11, 12. It is understood that also other configurations
of the front side of the first magnetic armature 11 and the second
magnetic armature 12 are conceivable to reduce the contact area 20,
to increase the holding force. This measure can be provided in
addition to arranging a permanent magnet.
[0038] FIG. 5 shows approximately the path of the magnetic field
lines around the permanent magnet 14 between the two magnetic
armatures 11, 12 when the coil is energized. It can be recognized
in particular that the permanent magnet 14 is magnetized in the
axial direction, to improve the holding force between the first and
the second magnetic armature 11, 12 and to optimally counteract the
force of the second biasing spring 10. The highest magnetic flux
density between the two magnetic armatures occurs at the contact
area of the two magnetic armatures (see FIG. 4).
* * * * *